If you choose to supply your own icon, It should be 48x48 pixels. You could also supply a 96x96 pixel icon, for high-resolution displays, and if you do this it will be specified as the 96 property of the icons object in manifest.json:

"icons":{"48":"icons/border-48.png",
"96": "icons/border-96.png"}

Alternatively, you could supply an SVG file here, and it will be scaled correctly.

borderify.js

Finally, create a file called "borderify.js" directly under the "borderify" directory. Give it this content:

document.body.style.border = "5px solid red";

This script will be loaded into the pages that match the pattern given in the content_scripts manifest.json key. The script has direct access to the document, just like scripts loaded by the page itself.

Testing it out

First, double check that you have the right files in the right places:

borderify/
icons/
border-48.png
borderify.js
manifest.json

Open "about:debugging" in Firefox, click "Load Temporary Add-on" and select any file in your add-on's directory:

The add-on will now be installed, and will stay until you restart Firefox.

Now try visiting a page under "mozilla.org", and you should see the red border round the page:

Try experimenting a bit. Edit the content script to change the color of the border, or do something else to the page content. Save the content script, then reload the add-on's files by clicking the "Reload" button in about:debugging. You can see the changes right away: